This invention relates to technology for generating a voltage for driving a DC-AC converter circuit in a discharge lamp lighting circuit.
A known construction of a lighting circuit for a discharge lamp such as a metal halide lamp has a DC power source, a DC-DC converter circuit, a DC-AC converter circuit and a starting circuit; the voltage and current of the discharge lamp are detected, and a control circuit is necessary for performing power control of the discharge lamp on the basis of corresponding detection signals.
If there is no fluctuation of the voltage from the DC power source, and power is supplied to the circuit at-the expected voltage value at all times, there is no problem; however, in practice it is necessary to stabilize the supplied voltage to remove voltage fluctuations.
Accordingly, methods such as generating a predetermined power supply voltage by raising or lowering the DC input voltage from the DC power source and supplying this to the control circuit and supplying it to a driving circuit (a driver IC or the like) of semiconductor switching devices constituting the DC-AC converter circuit have been employed. For example, when the DC input voltage from the DC power source has fallen, to keep the DC-AC converter circuit operating properly in order to maintain the lighting of the discharge lamp, it is necessary for this DC input voltage to be raised, and for this a stabilized power supply circuit using a transformer and switching devices and a drive control circuit thereof and a rectifying and smoothing circuit with diodes and capacitors has been provided so that the necessary voltage is obtained.
However, when the construction of the power supply circuit for obtaining the voltage for driving the DC-AC converter circuit becomes complex, the number of components constituting the power supply circuit increases, and there are consequent problems of increased cost and difficulty in achieving reductions in size and power consumption.
It is therefore an object of the present invention, in a discharge lamp lighting circuit having a DC-AC converter circuit, to simplify a circuit for generating the voltage for driving the DC-AC converter circuit and to reduce the cost and the power loss of the discharge lamp lighting circuit.
To achieve the above object and other objects, in the present invention, in a discharge lamp lighting circuit having a DC-DC converter circuit for converting an input voltage from a DC power source to a required DC voltage, a DC-AC converter circuit disposed down-circuit of the DC-DC converter circuit and having a bridge-type construction using a plurality of switching devices, and a control circuit for performing lighting control of a discharge lamp, a voltage generated from a power supply voltage to the control circuit is supplied as a voltage for driving the DC-AC converter circuit.
Accordingly, with this invention, because a generated voltage based on the power supply voltage to the control circuit is used as the voltage for driving the DC-AC converter circuit, it becomes unnecessary to generate the required voltage by raising or lowering directly the input voltage from the DC power source.